Methods are known for monitoring registration of images printed by a printer of the type in which at least two printing stations cause different images to be printed on a substrate, the printer being capable of registration adjustment. According to this method a first pattern of registration marks of a first color is printed onto the substrate by operation of one of the printing stations. A second pattern of registration marks of a second color is printed onto the substrate by operation of another printing station. The second pattern partially overlaps the first pattern to form a composite pattern of registration marks. The composite pattern is illuminated and the reflectivity thereof is examined to obtain a reflectivity signature for the composite pattern. The reflectivity signature of the composite pattern is compared with a predetermined signature to determine an adjustment factor for the printer.
In U.S. Pat. No. 5,402,726 (Levien) a register pattern and a system for bringing a pair of register patterns into alignment is disclosed. A first pattern comprises a plurality of dots of a first frequency, while a second pattern comprises a plurality of dots of a second frequency. When the patterns are overlaid, an interference pattern is observed. Images are in correct register when the interference pattern produces a maximum bright region in the center of the overlaid pattern. The position of the bright region is detected for example by the use of an on-line sensor of photosensitive elements. A small relative movement of the images out of register produces a relatively large movement in the position of the bright region. The position of the bright region indicates the direction and degree of correction required for correct registration.
The arrangement disclosed by Levien suffers from the disadvantage that a large number of dots have to be printed and measured in order to determine the position of the bright region and that it is necessary to calibrate an array of photosensitive elements. The Levien arrangement also requires a high measurement resolution in order to measure all of the composite pattern.
European patent application EP 0744669 (Xerox Corporation) discloses a wide area beam sensing method and apparatus for image registration calibration in a color printer. The method includes printing a first pattern of spaced registration marks in a first color onto a black or transparent image bearing member, printing a second pattern of spaced registration marks in black onto the image bearing member to partially overlap and mask the first pattern, illuminating the composite pattern so formed to produce actual diffuse and direct light reflectance measurement values from the printed marks and comparing the measurement values to obtain an adjustment factor for the printer.
This method suffers from the disadvantage that measurements have to be taken on a black or transparent image bearing surface, which is typically a transfer belt in the printer, whereas it is preferable to take measurements on the final image bearing substrate, typically paper, which is white or colored but almost certainly not black or transparent. Furthermore, it is preferable that measurements be taken after the toner image has been fixed to the substrate, whereas the method of EP 0744669 is essentially carried out prior to fixing the image. This is because the glossy effect of the fixed black toner will otherwise not have the necessary masking effect while fixing of a multicolor image results in some mixing of the different color toners resulting in an image density which may be different from that of the un-fixed image. Still further, the method of EP 744669 does not enable control of registration of two images printed in the same color, for example black on black, which may sometimes be required. Yet further, the method of EP 744669 does not enable control of registration of two images both printed in a color other than black or, for example, where "black" images are obtained by a superposition of images of three or more other colors.